The present invention relates to a method and apparatus for capturing a magnetic rotor for acceleration, rotation and deceleration with a rotating magnetic field generated by a stator for use in a turbogenerator including a compliant foil fluid film bearing.
Compliant foil fluid film radial bearings are currently being utilized in a variety of high speed rotor applications. These bearings are generally comprised of a bushing, a rotating element such as a rotor or a shaft adapted to rotate within the bushing, non-rotating compliant fluid foil members mounted within the bushing and enclosing the rotating element, and non-rotating compliant spring foil members mounted within the bushing underneath the non-rotating compliant fluid foil members. The space between the rotating element and the bushing is filled with fluid (usually air) which envelops the foils. Conventionally, the compliant fluid foil elements are divided into a plurality of individual compliant foils to form a plurality of wedge-shaped channels which converge in thickness in the direction of the rotation of the rotor.
The motion of the rotating element applies viscous drag forces to the fluid in the converging wedge channels. This results in increases in fluid pressure, especially near the trailing edge of the wedge channels. If the rotating element moves toward the non-rotating element, the convergence angle of the wedge channel increases, causing the fluid pressure rise along the channel to increase. Conversely, if the rotating element moves away, the pressure rise along the wedge channel decreases. Thus, the fluid in the wedge channel exerts restoring forces on the rotating element that vary with and stabilize running clearances and prevent contact between the rotating and non-rotating elements of the bearing. Flexing and sliding of the foils causes coulomb damping of any axial or overturning motion of the rotating element of the bearing.
Owing to preload spring forces or gravity forces, the rotating element of the bearing is typically in physical contact with the fluid foil members of the bearing at low rotational speeds. This physical contact results in bearing wear. It is only when the rotor speed is above what is termed the liftoff/touchdown speed that the fluid dynamic forces generated in the wedge channels assure a running gap between the rotating and non-rotating elements.
Compliant foil fluid film radial bearings typically rely on backing springs to preload the fluid foils against the relatively movable rotating element so as to control foil position/nesting and to establish foil dynamic stability. The bearing starting torque (which should ideally be zero) is directly proportional to these preload forces. These preload forces also significantly increase the rotor speed at which the hydrodynamic effects in the wedge channels are strong enough to lift the rotating element of the bearing out of physical contact with the non-rotating members of the bearing. These preload forces and the high liftoff/touchdown speeds result in significant bearing wear each time the rotor is started or stopped.
These compliant foil fluid film radial bearings (air bearings) may be positioned at multiple locations along the rotor in a turbogenerator assembly, such as between the stator, compressor, and turbine wheel. These air bearings are operative to support the shaft at rotational speeds above approximately 8,000 rpms. Rubbing of the rotor against the foil occurs prior to liftoff and after touchdown, which is generally under approximately 8,000 rpms. This rubbing is undesirable because it may cause wear on the foil.
While compliant foil fluid film radial bearings have been specifically described above, much the same considerations apply to compliant foil fluid film thrust bearings which are also currently being utilized in a variety of high speed rotor applications.
The least expensive turbogenerator design includes a sensorless rotor. By xe2x80x9csensorlessxe2x80x9d it is meant that the system includes no means for determining the rotational position of the rotor because the rotor has no sensors to provide such information. A problem inherent in such a low cost generator is that it may be difficult to capture the rotor for rotation with the stator if the rotational position of the rotor is unknown. The challenge is to not allow the rotor to rotate in contact with the air bearings for prolonged periods of time before liftoff has been reached. Such prolonged contact causes damage to the air bearings.
In order to achieve minimal contact between the shaft and the air bearings, quick acceleration is necessary, which requires efficient capture of the rotor for rotation with the stator. If the stator is simply accelerated quickly, it may fly by the rotor without capturing the rotor, in which case the stator must be decelerated again to capture the rotor, and then accelerated again. Obviously, this provides an inefficient system. Therefore, it is desirable to provide a method and apparatus for capturing, accelerating, and decelerating a sensorless magnetic rotor in a turbogenerator in a manner which minimizes the duration of contact between the rotor and the air bearings in order to prevent damage to the air bearings.
In accordance with one aspect of the invention, a method is provided for starting a turbogenerator having a sensorless magnetic rotor supported for rotation in a stator by a compliant foil fluid film bearing. The method comprises energizing the stator to generate a continuously rotating magnetic field to capture the magnetic rotor and thereafter accelerating the rotational speed of the magnetic field to substantially minimize the time required for the magnetic rotor to reach a liftoff speed associated with the compliant foil fluid film bearing.
In another aspect of the invention, the rotor is decelerated during shutdown and the inertial energy of the rotor is dissipated by converting the inertial energy to a DC bus voltage and dissipating the DC bus voltage with an off-load device including a brake resistor.